Measuring chemical properties of a sample fluid in dialysis systems

ABSTRACT

In one aspect of the invention, a method includes determining an amount of carbon dioxide (CO 2 ) in dialysate flowing through a dialysis system using a CO 2  sensor associated with the dialysis system, determining, using a pH sensor associated with the dialysis system, a pH level of the dialysate, and calculating a level of bicarbonate in the dialysate based at least in part on the determined amount of CO 2  measured in the gas and the determined pH level of the dialysate.

TECHNICAL FIELD

This invention relates to measuring chemical properties of a samplefluid in dialysis systems.

BACKGROUND

Dialysis is a treatment used to support a patient with insufficientrenal function. The two principal dialysis methods are hemodialysis andperitoneal dialysis.

During hemodialysis (“HD”), the patient's blood is passed through adialyzer of a dialysis machine while also passing a dialysis solution ordialysate through the dialyzer. A semi-permeable membrane in thedialyzer separates the blood from the dialysate within the dialyzer andallows diffusion and osmosis exchanges to take place between thedialysate and the blood stream. These exchanges across the membraneresult in the removal of waste products, including solutes like urea andcreatinine, from the blood. These exchanges also regulate the levels ofother substances, such as sodium and water, in the blood. In this way,the dialysis machine acts as an artificial kidney for cleansing theblood.

During peritoneal dialysis (“PD”), a patient's peritoneal cavity isperiodically infused with sterile aqueous solution, referred to as PDsolution or dialysate. The membranous lining of the patient's peritoneumacts as a natural semi-permeable membrane that allows diffusion andosmosis exchanges to take place between the solution and the bloodstream. These exchanges across the patient's peritoneum result in theremoval of waste products, including solutes like urea and creatinine,from the blood, and regulate the levels of other substances, such assodium and water, in the blood.

Many PD machines are designed to automatically infuse, dwell, and draindialysate to and from the patient's peritoneal cavity. The treatmenttypically lasts for several hours, often beginning with an initial draincycle to empty the peritoneal cavity of used or spent dialysate. Thesequence then proceeds through the succession of fill, dwell, and drainphases that follow one after the other. Each phase is called a cycle.

SUMMARY

In one aspect of the invention, a method includes determining an amountof carbon dioxide (CO₂) in dialysate flowing through a dialysis systemusing a CO₂ sensor associated with the dialysis system, determining,using a pH sensor associated with the dialysis system, a pH level of thedialysate, and calculating a level of bicarbonate in the dialysate basedat least in part on the determined amount of CO₂ measured in the gas andthe determined pH level of the dialysate.

In another aspect of the invention, a method includes extracting a firstportion of fluid from a fluid circuit of a dialysis system, causing thefirst portion of fluid to flow through a first channel into a firstchamber that contains a composition to liberate a first CO2 gas from thefirst portion of fluid, determining a level of total CO2 in the firstportion of fluid based at least in part on the first CO2 gas, extractinga second portion of fluid from the fluid circuit of the dialysis system,causing the second portion of the first portion of fluid to flow througha second channel into a second chamber to liberate a second CO2 gas fromthe second portion of fluid, and determining a level of total urea inthe second portion of fluid based at least in part on the second CO2gas.

In an additional aspect of the invention, a dialysis system includes adialysis machine including a pH sensor and a CO2 sensor, and a dialysisfluid chamber configured to be connected to the dialysis machine. Thedialysis fluid chamber includes a housing defining an inlet port, anoutlet port, a dialysis fluid passage extending between the inlet andoutlet ports, and first and second apertures adjacent the fluid passage,a pH reactive material disposed over the first aperture of the housing,and a gas-permeable membrane disposed over the second aperture of thehousing. The pH reactive material and the gas-permeable membrane alignwith the pH sensor and CO2 sensor, respectively, when the dialysis fluidchamber is connected to the dialysis machine such that the pH sensor andCO2 sensor can be used to detect a pH level and CO2 level, respectively,of a dialysis fluid flowing through the dialysis fluid chamber.

In an additional aspect of the invention, a dialysis fluid chamberincludes a housing defining an inlet port, an outlet port, a dialysisfluid passage extending between the inlet and outlet ports, and firstand second apertures adjacent the fluid passage, a pH reactive materialdisposed over the first aperture of the housing, and a gas-permeablemembrane disposed over the second aperture of the housing. The dialysisfluid chamber is configured such that the pH reactive material and thegas-permeable membrane align with a pH sensor and CO2 sensor,respectively, of a dialysis machine when the dialysis fluid chamber isconnected to the dialysis machine.

In an additional aspect of the invention, a dialysis system includes adialysis machine including a sensor, and a gas emission deviceconfigured to be connected to the dialysis machine in a manner such thatdialysis fluid can be forced into the gas emission device. The gasemission device includes a housing defining first and second chambers,and a member defining a first fluid passage leading to the first chamberand a second fluid passage leading to the second chamber, at least oneof the first and second fluid passages being heated such that dialysisfluid flowing along the at least one of the first and second fluidpassages is heated to a desired temperature. The first chamber containsan acid that causes CO₂ to be emitted from dialysis fluid that isdelivered to the first chamber via the first fluid passage, the secondchamber can cause a gas to be emitted from dialysis fluid that isdelivered to the second chamber via the second fluid passage, and thesensor is configured to detect an amount of CO₂ emitted from thedialysis fluid delivered to the first chamber and to detect an amount ofthe gas emitted from the dialysis fluid delivered to the second chamber.

In an additional aspect of the invention, a gas emission device isconfigured to be connected to a dialysis machine in a manner such thatdialysis fluid can be forced into the gas emission device. The gasemission device includes a housing defining first and second chambers,and a member defining a first fluid passage leading to the first chamberand a second fluid passage leading to the second chamber, at least oneof the first and second fluid passages being heated such that dialysisfluid flowing along the at least one of the first and second fluidpassages is heated to a desired temperature. The first chamber containsan acid that causes CO2 to be emitted from dialysis fluid that isdelivered to the first chamber via the first fluid passage, the secondchamber can cause a gas to be emitted from dialysis fluid that isdelivered to the second chamber via the second fluid passage, and thegas emission device defines a flute portion that is positioned adjacenta sensor of the dialysis machine when the gas emission device isconnected to the dialysis machine.

In an additional aspect of the invention, a method includes extracting afirst portion of fluid from a fluid circuit of a dialysis system,causing the first portion of fluid to flow through a first channel intoa first chamber that contains a composition to liberate a CO2 gas fromthe first portion of fluid, determining a level of total CO2 in thefirst portion of fluid based at least in part on the CO2 gas, extractinga second portion of fluid from the fluid circuit of the dialysis system,causing the second portion of the first portion of fluid to flow througha second channel into a second chamber to liberate a NH3 gas from thesecond portion of fluid, and determining a level of total urea in thesecond portion of fluid based at least in part on the NH3 gas.

Implementations can include one or more of the following features.

In some implementations, dialysate is caused to flow into a chamber thatincludes a gas-permeable membrane.

In some implementations, the membrane is configured to prevent liquidfrom passing through the membrane.

In some implementations, the CO₂ sensor includes an infrared sensor.

In some implementations, dialysate is caused to flow into a chamber thatincludes a material that is configured to alter an appearance of thematerial based at least in part on a pH level of the dialysate.

In some implementations, dialysate is caused to contact the material.

In some implementations, the material is configured to alter a color ofthe material based at least in part on the pH level of the dialysate.

In some implementations, the material includes a pH strip.

In some implementations, the material includes a sol-gel.

In some implementations, the pH sensor detects the alteration in theappearance of the material.

In some implementations, one or more artificial light sources are causedto direct light toward the material such that the material reflects atleast a portion of the directed light.

In some implementations, the pH sensor is used to detect at least aportion of light reflected by the material.

In some implementations, determining an amount of carbon dioxideincludes measuring an amount of CO₂ emitted from the dialysate.

In some implementations, determining an amount of carbon dioxide (CO2)in the dialysate includes determining a partial pressure of CO₂associated with a gas emitted by the dialysate.

In some implementations, calculating a net urea in the sample fluid isbased at least in part on a difference between the second CO₂ gas andthe first CO₂ gas.

In some implementations, the composition includes an acid.

In some implementations, the acid includes hydrochloric acid.

In some implementations, the composition is heated to a pre-definedtemperature.

In some implementations, the second chamber contains a urease.

In some implementations, the second chamber is heated to a desiredtemperature.

In some implementations, determining the level of total CO₂ in thesample fluid includes causing the first CO₂ gas to pass between a laserand a receiver configured to detect a beam emitted by the laser.

In some implementations, the beam is emitted at a wavelength thatoverlaps an absorption spectrum of CO₂ gas but does not overlap anabsorption spectrum of one or more of NH3 gas, acid gas, and watervapor.

In some implementations, determining the amount of total urea in thefluid includes causing the second CO₂ gas to pass between a laser and areceiver configured to detect a beam emitted by the laser.

In some implementations, the beam is emitted at a wavelength thatoverlaps an absorption spectrum of CO₂ gas but does not overlap anabsorption spectrum of one or more of NH3 gas, acid gas, and watervapor.

In some implementations, the first portion of the sample fluid or thesecond portion of the sample fluid is heated in the first or secondchannel, respectively, to liberate NH3 gas, and determining an amount ofNH3 in the sample fluid based at least in part on the NH3 gas.

In some implementations, determining an amount of NH3 in the samplefluid includes causing the NH3 gas to pass between a laser and areceiver configured to detect a beam emitted by the laser.

In some implementations, the beam is emitted at a wavelength thatoverlaps an absorption spectrum of NH3 gas but does not overlap anabsorption spectrum of CO₂ gas.

In some implementations, extracting at least one of the first and secondportions of fluid from the fluid circuit of the dialysis system includesusing a peristaltic pump to extract at least one of the first and secondportions of the fluid from a fluid line associated with the fluidcircuit.

In some implementations, the first and second portions of fluid includedialysate.

In some implementations, the first and second portions of fluid includeblood.

In some implementations, extracting the sample fluid does not interrupta dialysis treatment session being performed by the dialysis system.

In some implementations, the dialysis system is a hemodialysis system.

In some implementations, the first and second portions of fluid areextracted in a single extraction.

In some implementations, the second chamber contains a urease.

In some implementations, the second chamber is heated to a desiredtemperature.

In some implementations, the gas emitted from the dialysis fluiddelivered to the second chamber is CO₂.

In some implementations, the gas emitted from the dialysis fluiddelivered to the second chamber is NH3.

In some implementations, the dialysis machine further includes amicroprocessor in communication with the pH sensor and the CO₂ sensor,the microprocessor being programed to determine a level of bicarbonatein the dialysis fluid based at least in part on the detected pH and CO₂levels.

In some implementations, the dialysis machine further includes adialysis fluid inlet line that can be selectively placed in fluidcommunication with the first chamber or the second chamber, and a pumpconnected to the dialysis fluid inlet line, the pump being operable toforce fluid into the gas emission device via the dialysis fluid inletline.

In some implementations, the dialysis fluid inlet line is connected to adialysate line of the dialysis system such that dialysate can bedelivered to the gas emission device via the dialysis fluid inlet line.

In some implementations, the dialysis fluid inlet line is connected to ablood line of the dialysis system such that blood can be delivered tothe gas emission device via the dialysis fluid inlet line.

Other aspects, features, and advantages will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a hemodialysis system that includes ahemodialysis machine connected to a module with a sorbent device forrecycling spent dialysate.

FIG. 2 is a diagram of fluid flow in the dialysis system of FIG. 1.

FIG. 3 is a diagram of an apparatus of the dialysis system of FIG. 1that is used in measuring bicarbonate.

FIG. 4 is a diagram of the apparatus of FIG. 3 connected to thehemodialysis machine.

FIG. 5 is a diagram of an apparatus for measuring a pH level ofdialysate.

FIG. 6 illustrates a technique for determining a pH level of dialysate.

FIG. 7 is a diagram of an apparatus for measuring a pH level ofdialysate.

FIGS. 8A and 8B show side and top views, respectively, of a chemicalmeasurement system for use with dialysis machines.

FIG. 9 illustrates a process for measuring the chemical properties of asample fluid.

DETAILED DESCRIPTION

FIG. 1 shows a hemodialysis system 100 that includes a module 105fluidly coupled to a hemodialysis machine 110. The module 105 includes,among other things, a sorbent device holder 115 that holds a sorbentdevice 120. The module also includes a bicarbonate measurement unit 125that is connected to a manifold 130 of the module 105 via inlet andoutlet lines 132, 134. As will be described in greater detail below, themodule 105 is used to recycle spent dialysate so that the spentdialysate can be reused for hemodialysis treatment. During use of thehemodialysis system 100, dialysate is pumped from the module 105 to thehemodialysis machine 110. The dialysate is then passed through adialyzer 135 connected to the hemodialysis machine 110 at the same timethat a dialysis patient's blood is passed through the dialyzer 135. As aresult, toxins, such as urea, migrate across a permeable membrane (e.g.,hollow fibers) of the dialyzer 135 from the patient's blood to thedialysate, producing spent dialysate (i.e., dialysate that containstoxins removed from the patient's blood). The spent dialysate is pumpedto the module 105 where it passes through the sorbent device 120, whichremoves toxins from the spent dialysate. As a result of chemicalreactions that occur within the sorbent device 120, the recycleddialysate exiting the sorbent device 120 typically contains gas, such ascarbon dioxide. After exiting the sorbent device 120, the recycleddialysate travels into the module 105 and then is drawn into thebicarbonate measurement unit 125 via the inlet line 132, which isconnected to the manifold 130 of the module 105. The recycled dialysateis then forced from the bicarbonate measurement unit 125 back into themodule 105 via the outlet line 134, which is connected to the manifold130 of the module 105. The recycled dialysate is then cycled backthrough the dialysate circuit and reused to cleanse the dialysispatient's blood.

Certain desired substances (e.g., magnesium, calcium, potassium, andsodium) may be stripped from the dialysate as the dialysate passesthrough the sorbent device 120. Those stripped substances can be addedto the dialysate exiting the sorbent device 120 (e.g., prior to drawingthe dialysate into the bicarbonate measurement unit 125). As shown inFIG. 1, an infusate solution container 136 and a sodium chloridesolution container 138 are connected to a manifold 140 of the module 105via fluid lines 137 and 139, respectively. The infusate solution (e.g.,a solution including magnesium, calcium, and potassium) and sodiumchloride can be drawn into the dialysate flowing within the module 105by activating associated valves and pumps within the module 105. Themodule 105 may also include a bicarbonate container 191 that isconnected to the manifold 140 of the module 104 via fluid line 192.Using a process similar to that discussed above with regard to theinfusate solution and the sodium chloride, bicarbonate can be drawn intothe dialysate flowing within the module 105 by activating associatedvalves and pumps within the module 105.

As shown in FIG. 1, a dilution water container 141 is connected to thedialysis machine 110 via a fluid line 143. In some cases, certainsubstances, such as sodium, may be added to, rather than stripped from,the dialysate as the dialysate passes through the sorbent device 120. Asa result, the sodium concentration in the dialysate exiting the sorbentdevice 120 may exceed a maximum desired concentration. In such cases,dilution water can be added to dialysate that is exiting thehemodialysis machine 110 and flowing into the module 105 toward thesorbent device 120. The dilution water can be added to the dialysateexiting the hemodialysis machine 110 by activating a pump within thehemodialysis machine 110. Activating this pump draws the dilution waterfrom the dilution water container 141 and fluid line 143 into thedialysate exiting the hemodialysis machine 110 such that the sodiumconcentration of the dialysate exiting the hemodialysis machine 110 (andeventually flowing through the module 105) is reduced, as will bedescribed in greater detail below.

The sodium concentration of the dialysate passing through the dialyzer135 affects (e.g., increases or decreases) the sodium concentration inthe patient's blood. If the sodium concentration in the patient's bloodfalls outside a desired range, the patient may experience discomfort orillness. For this reason, a conductivity meter may be positioned withinthe module 105 to measure the conductivity of dialysate after thedialysate exits the sorbent device 120. These conductivity readings canbe used during treatment to determine the amount of sodium chloridesolution or dilution water to be added to the recycled dialysate exitingthe sorbent device 120. In particular, because the sodium in thedialysate is the predominant contributor to the conductivity of thedialysate, the sodium concentration of the dialysate can be determinedor approximated based on the conductivity readings. The amount of sodiumchloride solution or dilution water to add to the dialysate in order toachieve a desired sodium concentration within the dialysate can then bedetermined.

In addition to the manifolds 130 and 140, the module 105 includes amanifold 175 to which fluid lines 177, 179 extending from the bag 180are connected and a manifold 185 to which fluid lines 187, 189 extendingfrom an ammonium (NH4) sensor 190 are connected. The module 105 furtherincludes a manifold 200 by which a fresh dialysate container 202 and adrain container 203 are connected to the module 105 via a fluid line 204and a drain line 205, respectively. Each of manifolds 130, 140, 175,185, and 200 can, for example, include projections on which fluid linescan be positioned to connect the various components described above totheir respective manifold. Any of various other suitable connectionmechanisms can alternatively or additionally be used to connect thefluid lines to the manifolds.

The manifold 175 allows dialysate to be transferred from the module 105to the bag 180 and vice versa. In particular, using pumps and valveswithin the module 105, dialysate can be pumped into and suctioned out ofthe bag 180 via the fluid lines 177, 179 connected to the manifold 175.The manifold 185 permits dialysate to be transferred from the module 105to the ammonium sensor 190 and vice versa. By activating pumps andvalves within the module 105 in a desired manner, the dialysate can bepumped from the module 105 to the ammonium sensor 190 and can be drawnback to the module 105 from the ammonium sensor 190. By activating pumpsand valves within the module, fluid can be drawn into the module 105from the fresh dialysate container 202 via the fluid line 204, and fluidcan be pumped from the module 105 to the drain container 203 via thedrain line 205. With the sorbent device 120 positioned in the cartridgeholder 115, as shown in FIG. 1, fluid circulating within the module 105is allowed to pass through the sorbent device 120 to recycle thedialysate.

Still referring to FIG. 1, a blood component set 225 is secured to afront face of the hemodialysis machine 110. The blood component set 225includes arterial and venous patient lines 227, 229 that are connectedto a patient during treatment. The arterial patient line 227 isconnected to an inlet port of the dialyzer 135 via a series of bloodlines, and the venous patient line 229 is connected to an outlet port ofthe dialyzer 135 via a series of blood lines. A blood pump line 231positioned between the arterial patient line 227 and the dialyzer 135 isoperably connected to a peristaltic blood pump 230 extending from thefront face of the hemodialysis machine 110. The peristaltic blood pump230 can be operated to pump blood through the various blood lines andcomponents of the blood component set 225. In particular, operation ofthe blood pump 230 draws blood from the patient through the arterialpatient line 227. The blood continues through a series of blood linesand blood components (e.g., sensors) to the dialyzer 135. The bloodexits the dialyzer 135 and passes through another series of blood linesand components (e.g., sensors) and then is returned to the patient viathe venous patient line 229.

As the blood is pumped through the various blood lines and components ofthe blood component set 225, it may be desirable to inject certainsubstances, such as drugs and/or saline into the blood lines. As shownin FIG. 1, a drug vial (e.g., a heparin vial) 232 is connected to one ofthe blood lines via a drug delivery line 234. The drug delivery line 234is threaded through a peristaltic drug pump 236, which can be used todeliver the drug from the vial 232 to the blood circuit duringtreatment. A saline bag may also be connected to a blood line of theblood component set 225 via a priming line. This arrangement allowssaline to be delivered through the blood circuit formed by the bloodlines and components of the blood component set when desired.

In some examples, it is useful to measure an amount of bicarbonate indialysate while dialysis is being performed on a patient. In somesorbent-based dialysis systems, a bicarbonate concentration of thedialysate is established by the concentration of the fresh dialysate,and by the donation of bicarbonate from the sorbent device 120 (or othersources, such as the bicarbonate container 192) during treatment. Thebicarbonate in the premix is circulated through the sorbent device wheresome of the bicarbonate is broken down due to the acidity of the sorbentdevice 120. Additional bicarbonate is donated to the dialysate due tothe breakdown of a patient's urea in the sorbent device 120. Thesefactors complicate calculating or setting a fixed bicarbonateconcentration for the dialysate. Accordingly, it can be useful tomeasure the bicarbonate concentration of the dialysate during treatmentso that a bicarbonate concentration can be maintained during treatmentthat is appropriate for the patient.

One technique for determining the bicarbonate concentration of a fluidsuch as dialysate is defined by the Henderson-Hasselbach equation. TheHenderson-Hasselbach equation relates the bicarbonate concentration of afluid with the acidity (pH) of the fluid and the partial pressure ofcarbon dioxide (CO2) of the fluid. The Henderson-Hasselbach equation is:

${pH} = {6.1 + {\log \left( \frac{{H{CO}}\; 3}{0.03*{{Pa}{CO}}\; 2} \right)}}$

Thus, if two of the constituent concentrations of dialysate can bedetermined (e.g., the pH level and the partial pressure of CO2), thethird concentration (e.g., the bicarbonate concentration) can becalculated based on the known values.

FIG. 2 schematically illustrates the various components of the module105 connected to the hemodialysis machine 110. Referring to FIG. 2, amethod of performing hemodialysis will now be described. Prior tobeginning the dialysis treatment, fresh dialysate is drawn into themodule 105 from the fresh dialysate container 202 by selectivelyactivating a pump 241 and various valves of the module 105. The freshdialysate is then circulated through the module 105 by the pump 241.Prior to reaching the sorbent device 120, the dialysate passes through aflow meter 242 that is configured to measure the flow rate of thedialysate passing therethrough. A signal representing the flow rate ofthe dialysate can be transmitted from the flow meter 242 to a controlunit (e.g., a microprocessor). The control unit can use the detectedflow rate of the dialysate to control metering of the infusate solutioninto the dialysate.

As the dialysate passes through the sorbent device 120, certainsubstances, such as calcium, magnesium, potassium, and sodium may bestripped from the dialysate. As discussed above, the sorbent device 120is also adapted to remove toxins, such as urea, from fluid flowingtherethrough, but while the fresh dialysate from the fresh dialysatecontainer 202 would generally not contain any such toxins, it would havesome capacity to purify dialysate, allowing, for example, tap water tobe used as dialysate.

The infusate solution, which includes magnesium, calcium, and potassium,is then pumped into the fluid outlet line 275 from the infusate solutioncontainer 136 by activating a pump 244. As discussed above, the infusatesolution can be added to the dialysate to restore concentrations ofmagnesium, calcium, and potassium to desired levels. Maintaining theconcentration of these substances within the dialysis solution, such ascalcium, magnesium, potassium, and sodium, can help to prevent thepatient from experiencing discomfort during and after the treatment.

After introducing the infusate solution into the dialysate, the mixtureof the dialysate and infusate solution continues to flow through thefluid outlet line 275 and passes through the conductivity meter 145. Theconductivity meter 145 can estimate, based on the conductivity of thedialysate passing therethrough, the concentration of sodium within thedialysate. A pump 246 can then be activated in a manner to introducesodium chloride solution into the fluid outlet line 275 from the sodiumchloride solution container 138 if the conductivity reading indicatesthat the sodium level in the dialysate is lower than desired. The pump246 can be operated in a manner to meter a desired volume of sodiumchloride solution into the dialysate at a desired rate.

Similarly, a pump internal to the hemodialysis machine 110 can beactivated to inject dilution water (e.g., tap water) from the dilutionwater container 141 into the dialysate exiting the hemodialysis machine110 and entering the module 105 if the conductivity reading indicatesthat the sodium level in the dialysate is higher than desired. Thisdilution water pump can be operated in a manner to meter a desiredvolume of the dilution water into the dialysate at a desired flow rate.

A microprocessor (which may include the previously mentioned controlunit or a different processing device) is connected to the flow meter242, the conductivity meter 145, and the pumps 241, 244, 246, 256 and276. The microprocessor is also connected to the dilution water pumpinside the hemodialysis machine 110. The measured flow rate of thedialysate is transmitted in the form of a signal from the flow meter 242to the microprocessor. The microprocessor adjusts operation of the pumps241 and 256 based on the measured flow rate at the flow meter 242 toensure that a prescribed dialysate flow rate is achieved. Themicroprocessor also controls the pump 244 as a function of the flow rateof the dialysate measured by the flow meter 242. This arrangement helpsto ensure that a desired amount of the infusate is added to thedialysate, and thus helps to ensure a desired proportion of the infusateto the dialysate.

In response to receiving the signals from the conductivity meter 145,the microprocessor sends signals to the pumps 244 and 246 to cause someof the sodium chloride solution, if desired, to be introduced into thefluid outlet line 275. Similarly, in response to receiving these signalsfrom the conductivity meter 145, the microprocessor can cause thedilution water pump in the hemodialysis machine 110 to pump dilutionwater, if desired, into the dialysate exiting the hemodialysis machine110 and entering the module 105. As a result, the amount of sodiumchloride and/or dilution water delivered to the dialysate can betterachieve a desired sodium concentration within the dialysate (e.g., asodium concentration that closely matches the sodium concentrationprescribed by the dialysis patient's physician).

After passing through the conductivity meter 145, the dialysate passesthrough a check valve 254 and into the ammonium sensor 190, whichdetects ammonium levels within the dialysate.

After filling the bag 180 to a desired level with dialysate having adesired concentration of calcium, magnesium, potassium, and sodium, apump 256 is activated to draw the dialysate from the bag 180 into thehemodialysis machine 110 via fluid line 257. Before entering thehemodialysis machine 110, the dialysate may be caused to flow (e.g.,upon the activation of one or more valves) into the dialysate collectionunit 300 via the inlet line 132. The amount of bicarbonate in thedialysate is measured by the bicarbonate measurement unit 125 while thedialysate flows through the dialysate collection unit 300 (e.g., usingthe techniques described below), and is returned to the fluid line 257via the outline line 134.

Based on the amount of bicarbonate measured in the dialysate by thebicarbonate measurement unit 125, bicarbonate can be added to thedialysate from the bicarbonate container 191. For example, a pump 276can be activated which draws bicarbonate into the fluid line 275 via abicarbonate fluid line 192 from the bicarbonate container 191. Drawingbicarbonate into the fluid line 275 will alter the bicarbonate level ofthe dialysate, and can be continuously measured by the bicarbonatemeasurement unit 125 and adjusted until the desired level of bicarbonateis reached (e.g., a level of bicarbonate that is appropriate for apatient).

The dialysate is circulated through the hemodialysis machine 110 andpasses through the dialyzer 135 connected to the hemodialysis machine110. At the same time, a patient's blood is circulated through the bloodcomponent set 225, including the dialyzer 135, connected to thehemodialysis machine 110. As a result, toxins, such as urea, aretransferred across a permeable membrane (e.g., permeable microtubes) ofthe dialyzer 135 from the patient's blood to the dialysate. The spentdialysate exiting the dialyzer 135 is then routed back to the module105.

The spent dialysate passes through a fluid line 258 in the module 105.Depending on the desired volume of dialysate to be cycled back to thedialysis machine, some of the spent dialysate can be routed to a spentdialysate chamber of the bag 180 via open valve 260 while the remainderof spent dialysate is routed toward the sorbent device via open valve262. As a result of the dialysis, for example, fluid from the patientmay be added to the dialysate as the dialysate passes through thedialyzer 135. Thus, routing some of the spent dialysate to the bag 180can help to ensure that a substantially constant volume of dialysate iscirculated through the module 105 and/or the hemodialysis machine 110throughout treatment. The pump 241 along the fluid line 258 forces thevolume of the spent dialysate that is not routed to the bag 180 into thesorbent device 120 via the cartridge holder 115. As the spent dialysatepasses through the sorbent device 120, urea is removed from the spentdialysate. Calcium, magnesium, and potassium are also stripped from thespent dialysate by the sorbent device 120.

In the manner discussed above, after the recycled dialysate exits thesorbent device 120, the infusate solution is introduced into therecycled dialysate and, based on the conductivity reading at theconductivity meter 145, sodium chloride may be added to the recycleddialysate. Similarly, dilution water can be added to the spent dialysateexiting the hemodialysis machine 110 and entering the module 105 basedon the reading at the conductivity meter 145. In the initial stages oftreatment, sodium levels in the recycled dialysate tend to be lower thandesired due to the tendency of the sorbent device 120 to strip sodiumfrom the dialysate passing therethrough. Consequently, in the earlystages of the treatment, sodium chloride will typically be injected intoa fluid line to increase the concentration of sodium in the recycleddialysate. In later stages of the treatment, however, the sorbent device120 may contain high levels of sodium and thus release sodium into thespent dialysate as the spent dialysate passes through the sorbent device120. This can lead to higher than desired levels of sodium in therecycled dialysate passing through the fluid outlet line 134. In suchcases, dilution water is injected into the spent dialysate exiting thehemodialysis machine 110 and entering the module 105 to lower the sodiumconcentration of the spent dialysate. This spent dialysate then travelsthrough the module 105 to the sorbent device 120 where the dilutionwater and spent dialysate are filtered.

Injecting the dilution water into the spent dialysate before the spentdialysate passes through the sorbent device 120 to be filtered allowsthe use of tap water as the dilution water because the tap water will befiltered and purified as it passes through the sorbent device 120. Thisarrangement permits the hemodialysis system 100 to be operated with areadily available supply of dilution water and without the need forstoring large volumes of dilution water on site.

After flowing past the conductivity meter 145, the recycled dialysatepasses through the check valve 254 and into the ammonium sensor 190.After exiting the ammonium sensor 190, some of the recycled dialysate isrouted to the bag 180 and some of the recycled dialysate is routed tothe hemodialysis machine 110. The dialysate may again enter thebicarbonate measurement unit 125 via the inlet line 132 and a valve iffurther monitoring of the dialysate's bicarbonate level is desired. Themeasurements provided by the bicarbonate measurement unit 125 can beused to further alter the bicarbonate level of the dialysate (e.g., byintroducing additional bicarbonate from the bicarbonate container 191into the dialysate).

In order to ensure that an equal amount of fluid enters and exits thehemodialysis machine 110, a T-valve 264 is adapted to route a portion ofthe dialysate to the hemodialysis machine 110 via the fluid line 257 andto route excess dialysate to the fresh dialysate chamber of the bag 180.Because the flow rate of the dialysate at the T-valve 264 is generallygreater than the rate at which the dialysate is being pulled into thehemodialysis machine 110, there will typically be excess dialysatepassing through the T-valve 264 and that excess dialysate will be routedto the bag 180 where it is collected for later use.

The dialysate that is delivered to the hemodialysis machine 110 againpasses through the dialyzer where toxins are transferred from thepatient's blood to the dialysate. The spent dialysate is then routedback to the module and the process is repeated until a desired amount oftoxins has been removed from the patient's blood.

After completing the patient's treatment, the dialysate can be removedfrom the bag 180. For example, the pumps and valves of the module 105can be operated in a manner to pump the dialysate from the bag 180 intothe drain container 203 or into a plumbing drain. Emptying the bag 180can allow the user to more easily handle the bag 180 after treatment dueto the decreased weight.

After draining the bag 180 to a desired level, the external components(e.g., the sorbent device 120, the infusate container 136, thebicarbonate measurement device 125, the sodium chloride container 138,the bicarbonate container 192, the bag 180, the dialysate container 202,the drain container 203, and their associated fluid lines), which areconstructed as disposable, single use components, are disconnected fromthe module 105 and discarded.

Referring to FIG. 3, a dialysate collection unit 300 of the bicarbonatemeasurement unit 125 is shown that, when used in combination with one ormore sensors of the bicarbonate measurement unit 125, can measure the pHand the partial pressure of CO₂ of dialysate. In some examples, thedialysate collection unit 300 is a disposable component that can beremovably attached to the bicarbonate measurement unit 125. After the pHand the partial pressure of CO₂ of the dialysate have been determined,the Henderson-Hasselbach equation can be used to calculate a bicarbonateconcentration of dialysate as shown above.

The dialysate collection unit 300 includes a main body that is in fluidcommunication with the inlet line 132 that carries dialysate. Ingeneral, the main body of the dialysate collection unit 300 defines achamber which is adapted to receive dialysate from the inlet line 132.The main body is also in fluid communication with the outlet line 134that routes the dialysate back into the module 105. The inlet line 132and the outlet line 134 may be detachably connected to the dialysatecollection unit 300. In some examples, the main body is a cuvette (e.g.,a tube of circular or square cross section, made of plastic, glass, orfused quartz, that is designed to hold samples, especially forspectroscopic experiments).

A surface 303 of the main body of the dialysate collection unit 300defines an opening 305. The opening 305 is covered by a gas-permeablemembrane 311, such that dialysate flowing through the dialysatecollection unit 300 will flow over (but not through) both the opening305 and the gas-permeable membrane 311. In some examples, thegas-permeable membrane 311 is a breathable, waterproof fabric, such asGor-Tex®. The main body of the dialysate collection unit 300 alsoincludes one or more surfaces (e.g., surface 303) to which a pH-reactivematerial 309 has been applied in such a way that dialysate flowingthrough the dialysate collection unit 300 will contact the pH-reactivematerial 309. As described in further detail below, when the pH-reactivematerial 309 and the gas-permeable membrane 311 are aligned withappropriate sensors (e.g., sensors located on the module 105), thebicarbonate measurement unit 125 can be used to measure the amount ofbicarbonate in dialysate flowing through the main body of the dialysatecollection unit 300 without contacting the liquid dialysate with sensorsor probes.

The main body of the dialysate collection unit 300 is configured toreceive a flow of dialysate via the inlet line 132 and may partially orcompletely fill with dialysate. With dialysate flowing on one side ofthe gas-permeable membrane, gas emitted from the dialysate will passthrough the gas-permeable membrane 311, and can be detected and analyzedby one or more sensors as described below. Thus, dialysate flows intothe dialysate collection unit 300 via inlet line 132 and fills at leasta portion of the main body and exits the main body via the outlet line134. While the dialysate is in the main body of the dialysate collectionunit 300, the pH-reactive material 309 and the gas-permeable membrane311 can be used to determine the pH level and the partial pressure ofCO₂ of the dialysate, respectively. Examples of techniques and devicesused to calculate the pH level and partial pressure of CO₂ of thedialysate using, for example, the arrangement of FIG. 4 is discussedbelow.

FIG. 4 is a perspective view of the bicarbonate measurement unit 125 asused in the system of FIG. 1. In the example of FIG. 4, the dialysatecollection unit 300 is coupled to the module 105 via a couplingmechanism 409 that contacts one or more surfaces of the dialysatecollection unit 300 and the module 105. Examples of the couplingmechanism 409 include one or more snaps, latches, adhesives, hook andloop fasteners, magnets, and the like. The dialysate collection unit 300is coupled to the module 105 such that the gas-permeable membrane 311and the pH-reactive material 309 are aligned with a CO₂ sensor and a pHsensor, respectively. In the example of FIG. 4, the CO₂ sensor 403 is aninfrared gas sensor.

As described above, when dialysate flows through the main body of thedialysate collection unit 300, gas emitted from the dialysate istransferred through the gas-permeable membrane 311. The gas emitted fromthe dialysate is transferred into a gas collection chamber 405 of theCO₂ sensor 403, and may include both CO₂ gas as well as other gases.After the gas has passed through the gas-permeable membrane 311 and hasentered the gas collection chamber associated with the CO₂ sensor 403,the CO₂ sensor 403 can measure the concentration of CO₂ gas present inthe gas collected in the gas collection chamber 405. In some examples,the gas collection chamber forms a seal with the gas-permeable membrane311 and/or a surface of the dialysate collection unit 300 such that mostor all of the gas that has permeated the gas-permeable membrane 311passes directly into the gas collection chamber 405 of the CO₂ sensor403.

In some examples, the CO₂ sensor 403 is an infrared gas sensorconfigured to detect an amount of CO₂ in a fluid, such as the gascollected in gas collection chamber 405. The CO₂ sensor 403 may includean infrared source (e.g., a lamp or laser), a wavelength filter, and aninfrared detector. When the CO₂ gas enters the CO₂ measurement chamber,the gas concentration can be measured electro-optically by the gas'sabsorption of a specific wavelength in the infrared (IR). The IR lightis directed through the gas collection chamber 405 a detector associatedwith the CO₂ sensor. The IR light can also be reflected back toward adetector; that is, the detector does not necessarily need to bepositioned opposite the IR source. The detector may include an opticalfilter that eliminates all light except the wavelength that the selectedgas molecules can absorb, which allows the detector to measure theabsorption of the characteristic wavelength of light absorbed by the CO₂gas in the gas collection chamber 405. The CO₂ sensor 403 can then usethe collected information to determine the concentration of CO₂ in thegas collection chamber 405. The IR signal from the IR source can bechopped or modulated so that thermal background signals can be offsetfrom the desired signal.

According to Henry's law, the partial pressure of free CO₂ in a gas inequilibrium with (e.g., above) the dialysate is proportional to thedissolved CO₂ in the dialysate. That is, Henry's law describes theequilibrium between a vapor and a liquid. At a constant temperature,Henry's law states:

p=k_(H)c,

where p is the partial pressure of the solute in the gas in equilibriumwith the solution, c is the concentration of the solute and kH is aconstant with the dimensions of pressure divided by concentration. Theconstant, known as the Henry's law constant, depends on the solute, thesolvent and the temperature. The relationship between the solubility ofCO₂ and temperature is shown below.

Temperature (° C.) 0 10 20 30 40 50 80 100 Solubility 1.8 1.3 0.88 0.650.52 0.43 0.29 0.26 (cm3 CO₂/g water)Thus, using Henry's law in combination with the measured concentrationof CO₂ gas in equilibrium with the dialysate, the concentration ofdissolved CO₂ in the dialysate can be determined.

As shown by the Henderson-Hasselbach equation above, once the CO₂ of thedialysate has been determined, calculating the pH of the dialysate willyield the values that are necessary to calculate the bicarbonateconcentration of the dialysate. FIG. 5 represents an exemplary techniquefor determining a pH level of the dialysate.

The dialysate collection unit 300 is coupled to the module 105 such thatthe pH reactive material 309 is aligned with the pH sensor 401. Asdescribed above, when dialysate flows through the main body, thedialysate contacts and reacts with the pH reactive material 309 whichcan be a pH indicator strip. In general, a pH indicator strip is amaterial that changes color depending on the pH—the acidity oralkalinity—of a liquid. pH indicators are sometimes weak acids or weakbases that change color at specific pHs. For instance, methyl red is acommon indicator that is red at pH of 5 and yellow at a pH of 6.Indicators which are covalently bonded to the strip substrate can beused when using indicator strips to avoid contamination of thedialysate. In some examples, the pH reactive material 503 can be asol-gel that is applied to one or more inner surfaces of the pHmeasurement chamber 309. The sol-gel can be of a type that reacts withthe dialysate to change color in a manner similar to that of a pHindicator strip.

In some examples, the pH reactive material 309 is coupled to an inner,clear surface of the main body of the dialysate collection unit 300(e.g., a surface opposite the pH sensor 401, as shown in FIG. 4). Whenthe dialysate contacts the pH reactive material 309 for a sufficientlength of time, the pH reactive material 309 will change its color to acolor that represents the acidity or alkalinity of the dialysate. The pHsensor 401 is configured to detect the color state of the pH reactivematerial 309 and determine the acidity or alkalinity of the dialysatebased on the detected color state. An exemplary technique for using a pHsensor 401 in combination with a pH reactive material 309 to determinethe acidity or alkalinity of the dialysate is shown in FIG. 6 (discussedbelow).

FIG. 5 shows an example arrangement 500 for determining the pH level ofdialysate flowing through the main body of the dialysate collection unit300. The arrangement 500 includes the pH sensor 401, which includes aphototransistor 501, and red and blue LEDs 503A, 503B. Thephototransistor 501 is aligned with the main body of the dialysatecollection unit 300 to detect light reflected from the pH reactivematerial 309. Light from either the red LED 503A or the blue LED 503B isemitted toward the pH reactive material 309 through a clear surface ofthe main body such that the emitted light reflects off the pH reactivematerial 309. In some examples, the pH reactive material 309 has a“fuzzy” or “matte” surface such that light may be reflected regardlessof the angle. In some examples, the dialysate collection unit 300 ispositioned at an acute angle (e.g., 20 degrees) with respect to thephototransistor 501 to avoid reflecting light from a surface of the mainbody back onto the phototransistor 501.

FIG. 6 illustrates an example process 600 for using the pH sensingarrangement 500 to determine the pH of the dialysate. Referring to bothFIGS. 5 and 6, the process 600 begins when the red LED 503A emits redlight 505A toward the pH reactive material 309 (602). Thephototransistor 501 captures the light 507A reflected from the pHreactive material 309 (604). The blue LED 503B emits blue light 505Btoward the pH reactive material 309 (606). The phototransistor 501captures the light 507B reflected from the pH reactive material 309(608). A processor associated with the pH sensor 401 (e.g., the mainprocessor of the hemodialysis system 100) determines whether asufficient sample size has been obtained (610). If a sufficient samplesize has not been obtained (NO), the process 600 begins applying lightand capturing reflected light in the sequence described above. If asufficient sample size has been obtained (YES), the processor calculatesthe average red/blue ratio (e.g., the average amount of red light andblue light reflected by the pH reactive material 309). The amount ofblue or red light detected by the phototransistor 501 can be representedby the difference between an output value of the phototransistor 501when an LED is emitting light, and an output value of thephototransistor 501 when no LEDs are emitting light. It should be notedthat the process 600 need not follow the exact sequence described above.For example, the blue LED 503B may emit blue light 505B before the redLED 503A emits red light 505A. The average blue/red ratio can then beused to determine pH via an empirically determined nonlinear curve fit.If desired, lots of pH reactive material may be tested and calibrated,with calibration data included in the disposable. The calibration datamay be read by any suitable means including barcode, serial ROM or RFID.

FIG. 7 is an example pH sensing arrangement 700 for determining the pHof dialysate in contact with the pH reactive material 309. In thisexample, the pH sensor 401 includes a camera 701 aligned with the pHreactive material 309. When the pH reactive material 309 is contacted bythe dialysate and changes color, the camera 701 (e.g., a color camera)can optically capture the color state of the pH reactive material 309.Software, hardware, or a combination thereof associated with one or moreof the hemodialysis machine or the camera may then use the capturedcolor information to determine the pH level of the dialysate.

In some examples, it is also possible to measure bicarbonate levels orother chemical properties of a sample fluid (e.g., blood or dialysate)using other related techniques. FIGS. 8A and 8B show top and side viewsof a chemical measurement system 800 for use with dialysis machines.Briefly, by subjecting a sample fluid such as dialysate to one or moreconditions, chemical properties can be determined for one or more of thesample fluid and a second associated fluid (e.g., the blood of a patientwho is undergoing treatment).

In FIG. 8A, a sample of dialysate is extracted from a fluid path in thedialysis system. For example, a peristaltic (or “roller”) pump can beused to extract dialysate from the fluid line 258 (FIG. 2), near anoutlet of the dialyzer. After being extracted from the fluid circuit inthe dialysis system 100, the sample fluid can be drawn through a fluidline 802 into a measurement system 804. The fluid line 802 is arrangedto deposit the sample fluid onto a heater block 806. For example, thefluid line 802 could be positioned above the heater block 806 so thatsample fluid exiting the fluid line 802 falls onto the heater block 806.The heater block 806 can be positioned on top of a loadcell 810 tomeasure a precise quantity of sample fluid that has been transferred tothe measurement system 804 (e.g., by measuring the weight of the samplefluid). Additionally, the heater block could have a defined volumewherein overflow is discarded. The amount of sample fluid detected bythe load cell 810 can be compared with a displacement of the pump 801 toprovide safety redundancy.

Once the sample fluid has been drawn into the measurement system 804,the sample fluid can be subjected to a number of conditions in order toidentify one or more chemical properties of the sample fluid. Forexample, as described below, the measurement system 804 can be used todetermine the blood urea nitrogen (“BUN”), ammonia (NH₃), and totalcarbon dioxide (CO₂, as a sum of pCO₂ and bicarbonate) levels of thesample fluid. By liberating certain gases from the sample fluid usingone or more conditions such as heat, chemical compounds, changes inpressure, or a combination thereof, an optical detection system thatincludes a laser 820 and an optical detector 822 can be used to identifyproperties of the sample fluid based on the liberated gases that riseinto beams emitted by the laser 820. Other light sources and opticalfilters may be used in place of a laser.

As shown in FIG. 8B, the heater block 806 can include a first heatedchannel 812 and a second heated channel 814. The fluid line 802 can bemovably positioned over either the first heated channel 812 (in position“1”) or the second heated channel 814 (in position “2”) such that samplefluid exiting the fluid line 802 will fall into one of the heatedchannels 812, 814. In either position, the sample fluid can be heated asit travels down the heated channel (e.g., to approximately 95° C.) inorder to drive off NH₃ and some CO₂ from the sample fluid. Because thevapor pressure of ammonia is higher than that of water, heating thesample fluid in such a manner can drive off ammonia, liberating NH₄+ asNH₃ gas. The NH₃ gas, once liberated, travels up the chimney 805 intothe path of one or more beams 824, 826 emitted by the laser 820. Thedetector 822 can determine properties of the gas passing through a beam(e.g., beam 824) based on the light that passes through the gas. Thewavelength of the beams 824, 826 is based on absorption properties ofthe gases that are desired to be measured or excluded from measurement.Accordingly, if the beam 824 is emitted at a wavelength (e.g., 1.52 μm)that ignores the absorption factors of water vapor and/or other gases(e.g., CO₂ gas) and overlaps with an absorption range of NH₃, the amountof the beam 824 absorbed by the gas can be detected and measured by thedetector 822 in order to determine the properties of the gas (e.g., aconcentration of NH₃ in the gas). This process allows the measurementsystem 804 to determine the amount of blood ammonia in the sample fluid.If the sample fluid is dialysate, the amount of ammonia in the blood ofa patient undergoing dialysis can be determined based on the amount ofammonia in the dialysate.

The measurement system 804 can also be used to measure the total CO₂ ofthe sample fluid, where the total CO₂ represents the sum of pCO₂ andbicarbonate. In some examples, the fluid line 802 is arranged inposition “1” and deposits sample fluid into the heated channel 812. Thesample fluid travels down the heated channel 812 (which is formed in adeclined surface of the heater block 806) and is deposited in a firstchamber 816 of a container unit 808 that includes the first chamber 816and a second chamber 818. In some examples, the container unit 808 canbe a disposable unit that is discarded after a predetermined number ofuses or after a predetermined time of service. The first chamber 816contains an acid (e.g., diluted HCL) that has been heated to apredetermined temperature depending upon the desired reaction rate(e.g., 70 C). Solid acids may also be used with the advantage of notevaporating during the course of the treatment. Metal bicarbonates(e.g., NaHCO3) are decomposed by acid (e.g. NaHCO₃+HCL→NaCl+H₂CO₃,H₂CO₃→CO₂+H₂O), so depositing the sample fluid in the acid in the firstchamber 816 will liberate CO2 gas associated with the decomposition ofbicarbonates such as HCO₃—. Furthermore, because the vapor pressure ofCO₂ is higher than that of water, depositing the sample fluid in acid ata sufficiently high temperature will liberate substantially all of thedissolved CO2 from the sample fluid. Thus, the total CO₂(pCO₂+bicarbonate) of the sample fluid can be determined by measuringthe total CO₂ emitted from the sample fluid deposited into the firstchamber 816.

The liberated CO₂ gas travels up the chimney 805 into the path of thebeam 826 emitted by the laser 820. The beam 826 is emitted at awavelength (e.g., 2.10 μm) that overlaps an absorption spectrum of CO₂gas but does not overlap the absorption spectrums of one or more of NH₃gas, acid gas, and water vapor. Accordingly, in a manner similar to thatdiscussed above, the detector 822 can determine the level of CO₂ in thegas emitted from the sample fluid that was deposited into the firstchamber 816. The level of bicarbonate or pC0₂ associated with the samplefluid can also be determined by calculation. For example, the CO₂concentration detected may be integrated over the detection time to givetotal CO₂ emission in moles, giving moles of NaHCO₃ (1:1). The NaHCO₃concentration may be computed from a known weight or volume of fluid;however, other salts may also contribute CO₂.

The measurement system 804 can also be used to measure the level or ureain the sample fluid. In some examples, the fluid line 802 is arranged inposition “2” and deposits sample fluid into the heated channel 814. Thesample fluid travels down the heated channel 814 (which is formed in adeclined surface of the heater block 806) and is deposited in the secondchamber 818 of the container unit 808. This container includesconditions that can liberate gases associated with the urea content ofthe sample fluid. For example, the second chamber 818 can contain aurease (e.g., jack bean meal urease) that decomposes the urea in thesample fluid, resulting in CO₂ and NH₃ reaction product gases. Eitherthe CO₂ or the NH₃ reaction product gases can be measured using thelaser 820 and the detector 822 to provide the level of urea in thesample fluid. Depending which reaction product gas is selected to beused as an indication of the urea in the sample fluid, a laser that hasa wavelength overlapping an absorption factor of the selected reactionproduct gas is used to detect the properties of the gases emitted fromthe sample fluid.

FIG. 9 illustrates a process 900 for measuring the chemical propertiesof a sample fluid, such as dialysate or blood. A fluid sample isextracted from a dialysis system (902). In some examples, the fluid canbe extracted from the dialysis system without interrupting a patient'songoing treatment session. The fluid can be extracted using a variety oftechniques and systems, such as the roller pump arrangement illustratedin FIG. 8A. The amount of sample fluid extracted can be controlled atthe point of extraction, and the actual amount of sample fluid extractedfor measurement can be confirmed (e.g., using the loadcell 810 shown inFIG. 8A).

A portion of the sample fluid is caused to flow into a chamber toliberate a first CO₂ gas (904). For example, as shown in FIGS. 8A and8B, the sample fluid (or a portion thereof) may be deposited in a heatedchannel that leads to a first chamber at the base of an inclinedsurface. The sample fluid can be heated to liberate one or more gaseswhile the sample fluid travels toward the first chamber. In someexamples, these gases (e.g., NH₃) can be used to measure an amount ofammonia in the sample fluid. The first chamber contains conditions thatliberate CO₂ gas from the sample fluid. For example, the first chambermay contain a heated acid (e.g., diluted HCl) that liberates both watervapor and CO₂ from the sample fluid. The amount of urease provided inthe first chamber should be sufficient to resist contaminates for theduration of the desired testing period (e.g., one chemical level test,one dialysis treatment session, or longer). The liberated gases candrift upward toward a measurement point (e.g., the liberated gases candrift up a chimney such that they pass through one or more laser beamsthat traverse an opening of the chimney).

A level of total CO₂ in the sample fluid is determined based at least inpart on the first CO₂ gas (906). For example, using the laser detectionarrangement shown in FIG. 8A, a measurement system (e.g., measurementsystem 804) can determine the total level of CO₂ in the sample fluid.The total level of CO₂ represents a sum of pCO₂ and bicarbonate in thesample fluid.

A portion of the sample fluid is caused to enter a second chamber toliberate a second CO₂ gas (908). For example, the fluid line 802 can beactuated to a second position in order to deposit a second portion ofthe sample fluid (or all of the sample fluid) into a second heatedchannel that leads to a second chamber, or both chambers may be on amovable stage. The sample fluid can be heated to liberate one or moregases while the sample fluid travels toward the second chamber. Thesecond chamber includes conditions to liberate gases from the samplefluid. For example, the second chamber can include a urease (e.g., jackbean meal urease) that decomposes urea to liberate a product CO₂ gas.The amount of reactant provided in the second chamber should besufficient to resist contaminates for the duration of the desiredtesting period (e.g., one chemical level test, one dialysis treatmentsession, or longer). The second chamber may contain other conditions,such as heat and/or pressure, which may also interact with the samplefluid to liberate a product gas. The liberated gases can drift upwardtoward a measurement point (e.g., the liberated gases can drift up achimney such that they pass through one or more laser beams thattraverse an opening of the chimney). While CO₂ is used in this exampleas the product gas that will be measured, other gases could also beselected to serve as an indicator of the amount of urea in the samplefluid. For example, NH₃ can also be liberated in the second chamberusing jack bean meal urease and can be measured to determine the amountof urea in the sample fluid.

A level of total urea in the sample fluid is determined based at leastin part on the second CO₂ gas (910). For example, using the laserdetection arrangement illustrated in FIG. 8A, the measurement system 804can determine a level of total urea in the sample fluid based on a levelof CO₂ gas liberated by the interaction between the sample fluid and theconditions in the second chamber.

After one or both of the total CO2 and the total urea levels have beendetermined, adjustments can be made to the chemical composition ofdialysate within the dialysis system. These adjustments can also be madeto affect changes in the blood chemistry of a patient undergoingdialysis treatment.

In some examples, the measurement system 804 can be used to measure anamount of ammonium (NH₄+) in a sample fluid using a slightly differentconfiguration. A hydrophobic membrane can be arranged in a wall of afluid line containing the sample fluid, with a sealed chamber located onthe opposite side of the membrane. Using this configuration, gasconcentration will reach an equilibrium in the fluid and in the chamberon the other side of the membrane and can thus be measured by anysuitable detection mechanism, such as those described above with regardto FIGS. 1-8. Alternatively, NH₄+ could also be measured by liberatingthe ammonium with an alkali solution. The alkali would stabilize thedissolved bicarbonate and produce NH₃ as a reaction product gas. Urea inthe sample fluid would not distort the measurement of NH₄+, as urea isgenerally not reactive in the mild conditions described that would serveto liberate ammonia.

In some examples, if the sample fluid is dialysate, determining thechemical properties of the dialysate using the techniques describedabove can allow for the determination of certain aspects of a patient'sblood chemistry. For example, if the total CO₂, NH₃, and urea aremeasured in a sample of dialysate, the total CO₂, NH₃, and urea can bedetermined for the patient's blood as well based at least in part on themeasured properties of the sample dialysate.

In some examples, the extraction point for the sample fluid can affectwhat inferences may be drawn from the chemical properties measured inthe sample fluid. For example, if dialysate is extracted near the outputof a dialyzer, properties of the extracted dialysate can be used toinfer chemical properties of a patient's blood. If the dialysate isextracted near the input of a dialyzer, the chemical properties of thedialysate can be used to infer a measure of the efficiency of toxinremoval or the correct infusion of replacement substances such asbicarbonate. Regardless of the extraction point, adjustments in thechemical composition of the dialysate can be made based on the measuredchemical properties of the extracted sample fluid.

While the condition that causes the decomposition of urea in the secondchamber 818 has been described as a urease, applying a threshold levelof heat and/or pressure to the sample fluid can also cause theliberation of product gases that can be used to determine the amount ofurea in the sample fluid.

While the examples above describe the use of a laser 822 and a detector824 to measure the level of CO₂ in a gas, other types of CO₂ sensors canbe used to accomplish a similar effect.

While the examples above describe the pH sensor 401 as a camera or aphototransistor, the pH sensor 401 can also include one or morecolorimeteric sensors. In general, a colorimeteric sensor is asemiconductor chip that senses color.

While certain implementations have been described, other implementationsare possible.

In some implementations, the module 105 alternatively or additionallyincludes conductivity meters positioned slightly upstream of the sodiumchloride container 138 and/or slightly upstream of the infusate solutioncontainer 136. These conductivity meters can be used to control theamounts of sodium chloride solution and/or infusate solution deliveredto the fluid passing through the fluid outlet line 134.

Blood can also be extracted as a sample fluid from the blood componentset 225 (or from another component) via a pump, and techniques similarto those described above can be used to determine bicarbonate levels ofthe blood, as well as other blood chemistry information. Blood can alsobe used as a sample fluid to determine chemical properties of dialysatewithin a dialysis system using techniques similar to those describedabove. An ultrafilter or plasmafilter may also be used to removemacromolecules and cellular materials to prevent fouling of the chemicalanalysis system and to assure that the CO₂ content measured is in theplasma, and not in the red cells.

While the external components (e.g., the sorbent device 120, thebicarbonate measurement unit 125, the bicarbonate container 191, theinfusate container 136, the sodium chloride container 138, the bag 180,the dialysate bag 202, the drain container 203, and their associatedfluid lines) connected to the module 105 have been described as beingdisposable, single use disposable components, any of these componentscan alternatively be reusable. For example, they can be constructed towithstand disinfection techniques, such as chlorine bleach rinses and/orother chemical rinses.

While the systems described herein have been described as includingdialysate recycling modules that are connected to the dialysis machine110, other arrangements are possible. In some implementations, forexample, the various components of the module are incorporated into asingle dialysis machine.

While the hemodialysis system 100 is configured so that dilution wateris introduced into the dialysate before the dialysate reaches thesorbent device 120 and sodium chloride solution is introduced into thedialysate after the dialysate exits the sorbent device 120, otherarrangements are possible. In certain implementations, for example, thesystem is configured such that the dilution water and sodium chloridesolution are both introduced to the dialysate before the dialysateenters the sorbent device 120. The pumps, pump lines, and line segmentsassociated with the delivery of the dilution water container 141 and thesodium chloride solution container 138 can, for example, be reconfiguredto deliver the dilution water and sodium chloride solution to theflowing dialysate. Alternatively, lines extending from the dilutionwater container 141 and the sodium chloride solution container 138 canbe connected to a common line via an actuated three-way valve. Thethree-way valve can be actuated in a manner so that as the pumpassociated with the pump line is operated dilution water, sodiumchloride solution, or no liquid is delivered to the dialysate via thecommon line.

While the systems described above are configured to deliver dilutionwater (e.g., tap water) to dialysate before the dialysate enters thesorbent device 120 (i.e., at a pre-sorbent device location), any of thesystems described herein can alternatively or additionally be configuredso that dilution water is introduced to dialysate after the dialysateexits the sorbent device 120 (i.e., at a post-sorbent device location).In such implementations, the dilution water would not pass through thesorbent device 120 before being delivered to the dialyzer 135.Therefore, the dilution water in such implementations would typically bea pre-filtered or purified water, such as AAMI water.

While the systems described above use the sorbent device 120 to removetoxins from the spent dialysate, other types of devices canalternatively or additionally be used to remove toxins from the spentdialysate.

While the systems describe above describe the bicarbonate measurementunit 125 being positioned to receive dialysate just before the dialysateenters the hemodialysis machine 110, the bicarbonate measurement unit125 can be positioned at other locations along the fluid path of thedialysate. Additionally, one or more additional bicarbonate measurementunits may be provided to measure levels of bicarbonate at other pointswithin the dialysis system 100. For example, an additional bicarbonatemeasurement device 125 could draw dialysate from the fluid line 258(FIG. 2) to measure the level of bicarbonate in dialysate exiting thehemodialysis machine 110. A difference between the bicarbonate measuredat fluid line 257 and fluid line 258 could then be calculated in orderto determine the patient disturbance of the bicarbonate level of thedialysate (e.g., to determine the effect of a patient's blood on thedialysate). The calculated patient disturbance could be used to furtheralter the bicarbonate levels of the dialysate. Other chemical levels(such as total CO₂, NH₃, NH₄+, and/or urea/BUN) can also be alteredbased on the chemical levels detected using the above-mentionedtechniques.

What is claimed is:
 1. A method comprising: determining an amount ofcarbon dioxide (CO₂) in dialysate flowing through a dialysis systemusing a CO₂ sensor associated with the dialysis system; determining,using a pH sensor associated with the dialysis system, a pH level of thedialysate; and calculating a level of bicarbonate in the dialysate basedat least in part on the determined amount of CO₂ measured in the gas andthe determined pH level of the dialysate.